1. Field of the Invention
This invention pertains to an apparatus and method for determining the height of a liquid above a reference level.
2. Description of the Prior Art
Various techniques have been employed in the prior art to measure the level of a liquid or the interface levels between two or more liquids. Mechanical devices such as floats are not sufficiently accurate for most applications. Devices using sonic or ultrasonic signals become complex when used to measure the surface levels of contained liquids because of echoes received from the surfaces of the supporting container. A prior art liquid level detection system which overcomes the deficiencies of mechanical and sonic systems is disclosed in U.S. Pat. No. 3,832,900 issued Sept. 3, 1974 to G. F. Ross and assigned to the assignee of the present invention. This invention employs an open coaxial transmission line which is immersed in a liquid. The surface of the liquid creates a discontinuity in the coaxial line which produces reflections of the incident pulses that propagate back along the coaxial line. The time difference between the received reflected pulse and the transmitted pulse determines the liquid level. However, the open coaxial line tends to clog, and requires frequent cleaning. Another prior art liquid level detection system is disclosed in U.S. Pat. No. 3,995,212 issued Nov. 30, 1976 to G. F. Ross and also assigned to the assignee of the present invention. This invention utilizes a single wire transmission line, immersed in a liquid and overcomes the clogging problem. The prior art liquid level detection system utilizes reflected waves on a transmission line to provide an accurate liquid level measurement but suffers from being unable to measure the depth of electrically conductive liquids. The present invention overcomes this deficiency and provides an apparatus and method for measuring the level and depth of a liquid that may be either electrically conductive or non-conductive.